Discovery of a redshifted X-ray emission line in the symbiotic neutron star binary 4U 1700+24

We present the spectral analysis of an XMM-Newton observation of the X-ray binary 4U 1700+24, performed during an outburst in August 2002. The EPIC-PN spectrum above 1 keV can be modeled by a blackbody plus Comptonization model, as in previous observations. At lower energies, however, we detect a prominent soft excess, which we model with a broad Gaussian centered at ~ 0.5 keV. In the high resolution RGS spectrum we detect a single emission line, centered at 19.19^+0.05_-0.09 Å. We discuss two possible interpretations for this line: O VIII at redshift z= 0.012^+0.002_-0.004 or Ne IX at redshift z ~ 0.4.